1. Field of the Invention
The invention relates to sterling silver in general and hardened, corrosion resistant sterling silver in particular.
2. Prior Art
Sterling silver is, by definition, a silver alloy that comprises at least 92.5 percent silver, by weight. The remaining 7.5 percent of the alloy is often comprised of copper, but can be any variety of combinations of metals, resulting in sterlings with varied characteristics. However, one common characteristic of sterlings is that they are generally soft.
Sterlings commonly have a Vickers Scale hardness of about 65-75, “as cast.” Sterling pieces are often cast in gypsum molds. As soon as the mold has cooled enough for the investment to have solidified, the entire mold will be submerged in water, causing the mold to shatter, thereby releasing the cast piece. This will anneal the cast sterling, making it softer. Nonetheless, the inventor believes that such pieces will have an annealed hardness value close to 65-75 on the Vickers Scale, such that the as cast hardness and the annealed hardness will be comparable for many prior art sterlings. In any event, the term “as cast,” as used herein, is intended to encompass investment that is released from its mold by submerging the same into a water bath, while hot.
Depending upon the intended application, the relative softness of most sterlings may or may not be a drawback. However, in many jewelry applications, softness is a decided liability. Sterling silver is generally not used in the setting of precious stones because of the risk that the sterling may bend and the stone lost. Hinges, clasps, earring pins and chains are also typically not made of sterling because of its relative softness. Likewise, the softness of sterling can result in scratches in the finish of high wear items such as rings and bracelets.
Sterling silver can be buffed to a high shine. However, because of its softness, mechanical buffing can mar the finish of traditional sterlings.
Two common ways of increasing the hardness of many metals, including sterling silver, are work hardening and age hardening. Work hardening involves physically working the piece (i.e., bending it, rolling it, drawing it, etc.). Work hardening is generally not appropriate for most pieces that have been cast, as it would change the appearance of the pieces.
Age hardening involves heating the piece. It is suitable for use with cast pieces as they may be heated after casting is complete. However, age hardening has an obvious drawback in that it will increase the cost of manufacturing the piece.
An advantage of traditional sterlings is that they typically are capable of taking a highly lustrous white finish. However, a corresponding disadvantage is that traditional sterlings are quite susceptible to corrosion or tarnishing. Thus, to maintain the highly lustrous finish desired in most sterling pieces, frequent polishing is usually necessary, if the piece is used at all.
In view of the foregoing shortcomings in the prior art, an improved sterling silver alloy is desired meeting one or more of the following objectives.